Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

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2020

Virginia L. McGuire

Ronald C. Seanor

William H. Asquith

Anna M. Nottmeier

David C. Smith

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This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Publications of the US Geological Survey by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Virginia L. McGuire, Ronald C. Seanor, William H. Asquith, Anna M. Nottmeier, David C. Smith, Roland W. Tollett, Wade H. Kress, and Kellan R. Strauch Water Availability and Use Science Program

Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

National Aeronautics and Space Administration (NASA) Earth Observatory images by Lauren Dauphin, using March 17, 2017, Moderate Resolution Imaging Spectroradiometer (MODIS) data from NASA Earth Observing System Data and Information System/Land, Atmosphere Near real-time Capability for EOS (EOSDIS/ LANCE) and Global Imagery Browse Services (GIBS) | Worldview.

Pamphlet to accompany Scientific Investigations Map 3453

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior DAVID BERNHARDT, Secretary U.S. Geological Survey James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2020

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Suggested citation: McGuire, V.L., Seanor, R.C., Asquith, W.H., Nottmeier, A.M., Smith, D.C., Tollett, R.W., Kress, W.H., and Strauch, K.R., 2020, Altitude of the potentiometric surface in the Mississippi River Valley alluvial aquifer, spring 2018: U.S. Geological Survey Scientific Investigations Map 3453, 13 p., 5 sheets, https://doi.org/10.3133/ sim3453 .

Associated data for this publication: McGuire, V.L., Seanor, R.C., Asquith, W.H., Nottmeier, A.M., Smith, D.C., Tollett, R.W., Kress, W.H., and Strauch, K.R., 2020, Datasets used to map the potentiometric surface, Mississippi River Valley alluvial aquifer, spring 2018: U.S. Geological Survey data release, https://doi.org/10.5066/P992HD1R.

ISSN 2329-132X (online) iii

Acknowledgments

This map is the culmination of efforts by personnel from the Arkansas Department of Health, Arkansas Geological Survey, Arkansas Natural Resources Commission, Illinois Department of Agriculture, Illinois State Water Survey, Louisiana Department of Natural Resources, Louisiana Department of Transportation and Development, Mississippi Department of Environmental Quality, Yazoo Mississippi Delta Joint Water Management District, Missouri Department of Natural Resources, U.S. Department of Agriculture–Natural Resources Conservation Service, U.S. Army Corps of Engineers, and the U.S. Geological Survey’s Central Midwest, Lower Mississippi-Gulf, Oklahoma-Texas, and Nebraska Water Science Centers, who collected, compiled, organized, analyzed, and verified the groundwater- and surface-water-altitude data. In addition to the authors, who were primarily responsible for ensuring that the information contained in this report is accurate and complete, the following individuals contributed substantially to the review of the water-level data and related potentiometric-surface map:

• Mississippi Department of Environmental Quality: James Hoffmann, Kay Whittington, Sam Mabry, Madison Kymes, and Pat Phillips;

• U.S. Department of Agriculture, Agricultural Research Service: James Robert (JR) Rigby; and

• Yazoo Mississippi Delta Joint Water Management District: David Kelly.

Contents

Acknowledgments ��iii Introduction ��1 Study Area Description ��3 Data and Methods ��3 Water-Level Data ��3 Characterizing the 2018 Potentiometric-Surface Raster and Contours ��8 Potentiometric-Surface Map, Spring 2018 ��9 Summary ��10 References Cited ��10

Figures

1. Map showing previous and current extent of the Mississippi River Valley alluvial aquifer and areas with insufficient groundwater-altitude data to map the potentiometric surface for spring 2018 ��2 2. Map showing location of wells with groundwater-altitude values and streamgages with surface-water-altitude values used to create the potentiometric-surface map of the Mississippi River Valley alluvial aquifer, spring 2018, and the part of the measurement month for the selected water-level-altitude value ��7

Tables

1. Total number of wells that were completed in the Mississippi River Valley alluvial aquifer and measured manually one or more times or continually from January through May 2018, and the subset of these wells whose groundwater-altitude data were used to generate the potentiometric-surface map for the Mississippi River Valley alluvial aquifer, spring 2018, by Mississippi Alluvial Plain region ��5 2. Summary statistics for water-level measurement dates of water levels used in the spring 2018 potentiometric-surface map for wells that were completed in the Mississippi River Valley alluvial aquifer and measured manually one or more times or continually as part of groundwater monitoring networks from January through May 2018, by Mississippi Alluvial Plain region ��6 3. Total number of streamgages in the Mississippi Alluvial Plain with surface-water-altitude values for April 10, 2018, and number of surface-water-altitude values, generally for April 10, 2018, used to generate the potentiometric-surface map, spring 2018, for the Mississippi River Valley alluvial aquifer by Mississippi Alluvial Plain region ��8 vi

Conversion Factors

U.S. customary units to International System of Units

Multiply By To obtain Length foot (ft) 0.3048 meter (m) mile (mi) 1.609 kilometer (km) Area section (640 acres or 1 square mile) 259.0 square hectometer (hm2) square mile (mi2) 259.0 hectare (ha) square mile (mi2) 2.590 square kilometer (km2) Flow rate million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Datum

Horizontal coordinate information is referenced to the World Geodetic Survey of 1984 (WGS 84). Historical data collected and stored as North American Datum of 1927 (NAD 27) or the North American Datum of 1983 (NAD 83) have been converted to WGS 84 for use in this publication. Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Historical data collected and stored as National Geodetic Vertical Datum of 1929 (NGVD 29) have been converted to NAVD 88 for use in this publication. Altitude, as used in this report, refers to distance above the vertical datum.

Abbreviations

GIS geographic information system MAP Mississippi Alluvial Plain MRVA Mississippi River Valley alluvial RMSE root mean square error USGS U.S. Geological Survey Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

By Virginia L. McGuire, Ronald C. Seanor, William H. Asquith, Anna M. Nottmeier, David C. Smith, Roland W. Tollett, Wade H. Kress, and Kellan R. Strauch

Previously published potentiometric-surface maps for a Introduction large part of the MRVA aquifer include maps from water levels measured from 1953 to 1961 (Krinitzsky and Wire, 1964), for The Mississippi River Valley alluvial (MRVA) aquifer 1964 (Boswell and others, 1968), and for 2016 (McGuire and is an important surficial aquifer in the Mississippi Alluvial others, 2019). Previously published potentiometric-surface Plain (MAP) area. The aquifer is generally considered to be maps for parts of the MRVA aquifer include maps for the an unconfined aquifer fig.( 1; Clark and others, 2011), and Grand Prairie region in Arkansas in 1929, 1939, and 1959 withdrawals are primarily used for irrigation (Maupin and (Engler and others, 1963) and selected counties in northeast Barber, 2005). These groundwater withdrawals have resulted and central Arkansas in 1965 and 1966 (Albin and others, in substantial areas of water-level decline in parts of the aqui- 1967; Plebuch and Hines, 1967); the entire aquifer area in fer. Concerns about water-level declines and the sustainability Arkansas for 1972, 1980, 1983, 1984, 1985, 1986, 1987, of the MRVA aquifer have prompted the U.S. Geological 1989, 1992, 1994, 1996, 1998, 2000, 2002, 2004, 2006, 2008, Survey (USGS), as part of the USGS Water Availability and and 2012 (Ackerman, 1989; Edds and Fitzpatrick, 1984; Use Science Program and with assistance from other Federal, Plafcan and Edds, 1986; Plafcan and Fugitt, 1987; Plafcan and State, and local agencies, to undertake a regional water- Remsing, 1989; Westerfield, 1990; Westerfield and Gonthier, availability study to assess the characteristics of the MRVA 1993; Westerfield and Poynter, 1994; Stanton and others, aquifer, including the potentiometric-surface altitude of the 1998; Joseph, 1999; Reed, 2004; Schrader, 2001, 2006, 2008, MRVA aquifer for spring 2018, and to provide information 2010, 2015); the aquifer area in northwestern Mississippi for to water managers to inform their decisions about resource various years including 1976, 1980, 1981, 1982, and 1983 allocations and aquifer sustainability. (Dalsin, 1978; Wasson, 1980, Darden, 1981, 1982a, 1982b, The purpose of this report was to present a 1983; Sumner, 1984, 1985; James Hoffmann, Mississippi potentiometric-surface map for the MRVA aquifer using manu- Department of Environmental Quality, written commun., ally measured groundwater-altitude data and daily mean or 2018); the entire aquifer area in Missouri for 1976 (Miller maximum groundwater-altitude data from wells measured and Appel, 1997); and the part of the aquifer in northeastern generally in spring 2018, which is after water levels have Louisiana for 1990 (Seanor and Smoot, 1995). The previ- substantially recovered from pumping in the previous irriga- ously published potentiometric-surface maps that were used tion season and before pumping begins for the next irrigation in this study were Miller and Appel (1997), Seanor and Smoot season, and using the altitude of the top of the water surface (1995), Schrader (2015), and McGuire and others (2019). in rivers in the area, hereinafter referred to as “surface-water To best reflect hydrologic conditions in the MRVA altitude,” generally on April 10, 2018, from streamgages in the aquifer, the groundwater altitudes used to create the 2018 area. The term “potentiometric surface” is used in this report potentiometric-surface map would be measured in a short because it is applicable for maps of the groundwater-altitude timeframe of days or 1 or 2 week(s) and there would be avail- surface in unconfined, semiconfined, and confined aquifers able data (for example, from sets of wells, with short [5 to (Lohman, 1972). In this report, the maps of the MRVA aqui- 10 feet (ft)] screens, installed near the top, in the middle, fer’s groundwater surface are termed potentiometric-surface and near the bottom of the aquifer) to indicate vertical flow maps as opposed to water-table maps because, although the components (Freeze and Cherry, 1979; Fetter, 2001). In fact, MRVA aquifer generally exhibits characteristics of unconfined the measurement timing for many wells was determined by the conditions, where surface-water features may or may not be needs and schedules of the entities doing the measurements hydraulically connected, it also exhibits characteristics of con- instead of the preferred schedule for a regional potentiometric- fined or semiconfined conditions in some areas at least during surface map, and many of the measured wells had longer part of the year. The location of these areas, where the aquifer (greater than 10 ft) screens, so these water-level measurements is confined or semiconfined, is not well understood or defined tend to represent an average hydraulic head in the aquifer for (Arthur, 1994; Kleiss and others, 2000). 2 Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

93 92 91 90 89 PLANATIN 55 ILLINOIS Mississippi Alluvial Plain MAP region 57 St. Francis 24 Cache 3 MISSOURI Grand Prairie KENTUCKY Delta Boeuf Atchafalaya e g Deltaic and Chenier Plains id R s y Urban areas e ARKANSAS l w 36 o r Areas of the MRVA aquifer with insufficient groundwater-altitude C 40 data, spring 2018 County or parish boundaryLabels shown for selected parishes Stream TENNESSEE MRVA aquifer extent previous 40 MRVA aquifer extent current 35 MISSISSIPPI

30 er iv R i p p i s s i 34 s s 530 i

55 Current extent of the MRVA aquifer 33 Atlantic Plain Division, Coastal Plain Province

MOREHOUSE PARISH 20 20

LOUISIANA OUACHITA PARISH 32 CATAHOULA PARISH

31 49 10 12 55 10 10

30 0 25 50 MILES

0 50 100 KILOMETERS

Previous extent of the MRVA aquifer (U.S. Geological Survey, 2015) GULF OF MEXICO Current extent of the MRVA aquifer (Painter and Westerman, 2018) Physiographic divisions (Fenneman and Johnson, 1946; U.S. Geological Survey, 2004) 29 MAP regions (Ladd and Travers, 2019) ase from U.S. Geological Survey digital data, 1:1,000,000 and 1:2,000,000 variously dated Highways from National Highway Planning Network, various scales, 2014 Urban areas from U.S. Census ureau, 1:500,000, 2016 Albers National Hydrogeologic Grid proection Standard parallels 4530’ N. and 2930’ N., central meridian 9600’ W. World Geodetic Survey of 1984

Figure 1. Previous and current extent of the Mississippi River Valley alluvial (MRVA) aquifer and areas with insufficient groundwater-altitude data to map the potentiometric surface for spring 2018. Data and Methods 3 that location (Fetter, 2001). For this report, recognizing the “Crowleys Ridge,” are erosional remnants of Tertiary-age limitations of the available data, it was decided to assess all deposits of clay, silt, sand, and lignite, overlaid by Quaternary- available groundwater-altitude data from wells measured from age sand and gravel, and capped by Quaternary-age loess January to May 2018 for use in the potentiometric-surface (fig. 1; Guccione and others, 1986; McFarland, 2004). The map for spring 2018. The resultant potentiometric-surface Crowleys Ridge areas are about 1,053 mi2; the combined map would then represent the generalized central tendency length of the two parts of Crowleys Ridge is about 185 mi for spring 2018, but it would not be useful for some purposes, and the width ranges from less than a mile to about 21 mi. such as for calibration of a groundwater-flow model for early Crowleys Ridge forms a physical barrier to groundwater flow April 2018 or for some local scale assessments. in the MRVA aquifer (Schrader, 2008, 2010, 2015; Kresse and others, 2014). Two other areas where the aquifer is not present are an upland area of about 128 mi2 in northeastern Louisiana in the center of Morehouse Parish and the northeastern part Study Area Description of Ouachita Parish, and an upland area of about 21 mi2 in the north-central part of Catahoula Parish (Saucier, 1994). The current (2019) extent of the MRVA aquifer is defined Groundwater withdrawals in 2000, from wells screened to be the same as the boundary of the MAP physiographic in the previously mapped extent of the MRVA aquifer (fig. 1; division, which is a revision of the aquifer extent used in pre- U.S. Geological Survey, 2015), averaged 9,290 million gallons vious studies (fig. 1; Ackerman, 1996; Clark and others, 2011; per day (Mgal/d), making it the third most heavily pumped Maupin and Barber, 2005; U.S. Geological Survey, 2015; aquifer in the Nation (Maupin and Barber, 2005). More than Painter and Westerman, 2018). The MRVA aquifer underlies 98 percent of total withdrawals in 2000, from wells completed 2 an area of approximately 43,800 square miles (mi ) in parts in the previously mapped extent of the aquifer, were for irriga- of seven States—Arkansas, Illinois, Kentucky, Louisiana, tion. Groundwater withdrawals associated with the revised Mississippi, Missouri, and Tennessee (fig. 1; Painter and extent (fig. 1; Painter and Westerman, 2018) of the MRVA Westerman, 2018). aquifer have not been determined. The MRVA aquifer primarily underlies the Mississippi Alluvial Plain Section within the Atlantic Plain Division, Coastal Plain Physiographic Province (fig. 1; Fenneman and Johnson, 1946; U.S. Geological Survey, 2004). The MRVA Data and Methods aquifer extends about 560 miles (mi) north to south from southeastern Missouri and Illinois and southwestern Kentucky The 2018 potentiometric-surface raster and associated to the Gulf of Mexico off Louisiana. The width of the MRVA contours were created by interpolating the groundwater- aquifer ranges from about 35 mi in northeast Louisiana and altitude data from wells and surface-water-altitude data from southwest Mississippi to 134 mi in southern Louisiana. The streamgages into a raster dataset (grid with a uniform cell size Mississippi River (fig. 1) is within the MRVA aquifer bound- [about 0.386 mi2] and hereinafter referred to as a “raster”) and ary except in southeast Louisiana, where the river is north of converting the resultant raster to contours using geographic the aquifer boundary and overlies aquifers in Pleistocene-aged information system (GIS) software (Esri® ArcMap, ver- deposits in Louisiana and not the MRVA aquifer (Smoot, sion 10.7; Esri, 2018) . The method used to create the 2018 1986). Where the Mississippi River is within the MRVA potentiometric-surface map is the same method used for the aquifer boundary, the river is along the eastern boundary of the 2016 potentiometric-surface map (McGuire and others, 2019). northern and southern part of the aquifer; in the central part The point shapefiles of groundwater- and surface-water- of the MRVA aquifer, the Mississippi River curves toward the altitude data, raster files of the potentiometric-surface map, middle of the aquifer at the northwest boundary of Mississippi and shapefile of the potentiometric-surface-altitude contours before curving back toward the eastern boundary of the aqui- are available from McGuire and others (2020). fer about 50 mi south of the northeast boundary of Louisiana (fig. 1). The MRVA aquifer is contained in Quaternary-age sand, Water-Level Data gravel, silt, and clay deposits overlying Tertiary-age units Groundwater-altitude data were compiled by the USGS (Hosman and Weiss, 1991; Saucier, 1994; Clark and oth- (table 1; McGuire and others, 2020) from 1,126 wells com- ers, 2011). In some areas, the MRVA aquifer is overlain by a pleted in the MRVA aquifer and measured either manually Quaternary-age confining unit of silt and clay; where present, or continually from January through May 2018. The wells this confining unit impedes recharge to the MRVA aquifer were measured as part of a regular water-level monitoring (Ackerman, 1989; Boswell and others, 1968; Kleiss and program by the Arkansas Natural Resources Commission, others, 2000). There are four areas within the MRVA aquifer Arkansas Department of Health, Arkansas Geological Survey, extent where the MRVA aquifer is not present (fig. 1; Painter Illinois Department of Agriculture, Illinois State Water Survey, and Westerman, 2018). The two northernmost areas, in north- Louisiana Department of Natural Resources, Louisiana eastern Arkansas and southeastern Missouri, which are termed Department of Transportation and Development, Mississippi 4 Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

Department of Environmental Quality, Missouri Department measurement was selected as the groundwater altitude to con- of Natural Resources, Yazoo Mississippi Delta Joint Water sider for use to create the potentiometric-surface map. For the Management District, U.S. Department of Agriculture-Natural manually measured wells with more than one measurement, Resources Conservation Service, and USGS. Wells measured the maximum (highest) groundwater altitude for each well was by drillers in Missouri were included in the data used to map selected; the difference between the maximum and minimum the potentiometric surface for the MRVA aquifer in 2016 groundwater-altitude values ranged from 0.00 to 13.86 ft, (McGuire and others, 2019) but were not included for the with a median difference of 1.03 ft. For the 28 wells measured 2018 potentiometric-surface map because the data from 2018 continually and used in the potentiometric-surface map, the were not yet available (July 2019). daily mean groundwater-altitude values for February 25, 2018 The groundwater altitude in wells that were manually (1 well); April 9, 2018 (1 well); or April 10, 2018 (26 wells) measured one or more times is hereinafter referred to as were used. Values for wells measured continually on dates “manually measured.” The groundwater altitude in wells other than April 10, 2018, were used when measurements were that were measured continually for all or part of the time not available for April 10, 2018. period is hereinafter referred to as “continually measured.” The distribution of measurement dates for water levels The manually and continually measured water levels were ranges from 0 wells for the first 15 days in January 2018 stored in the USGS National Water Information System to 706 wells in the first 15 days of April 2018 (fig. 2). This database (U.S. Geological Survey, 2019a, 2019b) as depth to distribution indicates that if only wells measured in a short water below land surface. For the manually and continually timeframe, such as the first 15 days in April 2018, were used measured wells, the land-surface altitude, in feet, and to create the 2018 potentiometric-surface map, there would associated vertical datum (National Geodetic Vertical Datum be larger areas with no groundwater-altitude data (fig. 2; of 1929 [NGVD 29] or North American Vertical Datum of table 2). To partially assess the variability in water levels by 1988 [NAVD 88]) were retrieved or determined for each well. measurement date, the difference for each MAP region in If the stored land-surface altitude datum was NGVD 29, the the mean of the daily mean water-level measurements for land-surface altitude was converted to NAVD 88 using the the continually measured wells for February 10, 2018, were National Geodetic Survey’s VERTCON computer program compared to the value for April 10, 2018, with the exception (Miller, 1999), and the measured groundwater altitude or of one well that was not measured on February 10, 2018. The daily mean groundwater altitude with respect to the NAVD 88 results were St. Francis region, 6 wells, −3 ft; Cache region, was calculated for each well. Groundwater altitudes from the 6 of the 7 wells, −3 ft; Grand Prairie region, 4 wells, 0 ft; well’s measuring point for manually and continually measured Delta region, 9 wells, −3 ft; Boeuf region, 2 wells, 0 ft; and for wells are assumed accurate to the hundredths of a foot. 27 of the 28 wells, −2 ft. These results indicate that water-level All groundwater-altitude data from manually and con- altitude on February 10 in the wells was approximately the tinually measured wells were reviewed to identify and exclude same or lower than the water-level altitude on April 10. groundwater-altitude values that appeared to be affected by Daily mean surface-water-altitude data were assembled current or recent pumping and that were substantially differ- for 104 streamgages routinely operated by the U.S. Army ent from the groundwater altitudes in nearby wells, possibly Corps of Engineers and the USGS in the MRVA aquifer area because of local or seasonal conditions. Other considerations (table 3; U.S. Geological Survey, 2018; U.S. Army Corps for rejecting a well’s groundwater altitude were apparent of Engineers, 2018; McGuire and others, 2020). For this discrepancies between the spatial location of the well and the study, the streamgage altitude, in feet; the associated vertical well’s legal description or identifier and suspected inaccuracy datum (NGVD 29 or NAVD 88); and the daily mean river in the land-surface altitude value. In addition, groundwater- stage on April 10, 2018, were retrieved for each streamgage. altitude data from wells were not used for (1) wells that were If the vertical datum associated with the streamgage altitude flowing and could not be measured or (2) wells that were was NGVD 29, the streamgage altitude was converted to dry. Groundwater-altitude data from 46 of 1,144 manually NAVD 88 using National Geodetic Survey’s VERTCON measured wells and 2 of 30 continually measured wells were program (Miller, 1999) for possible use to create the not used in the 2018 potentiometric-surface map (table 1; potentiometric-surface map. McGuire and others, 2020). Of the 104 streamgages considered for use in the Groundwater-altitude data from 1,126 wells were used potentiometric-surface map (table 3), 25 were in areas with in the spring 2018 potentiometric-surface map. These wells little or no groundwater data (defined as no groundwater included 1,052 manually measured wells, which were mea- data within 12.4 mi of the center of the cell that contains the sured one time; 46 manually measured wells, which were mea- streamgages); 12 had surface-water altitudes that were much sured two or four times; and 28 continually measured wells higher than the nearby wells screened in the MRVA aquifer, that were in operation for all or part of the time from January likely either because the surface-water altitude was affected to May 2018 (table 1; McGuire and others, 2020). The median by precipitation events or the MRVA aquifer is not connected measurement date for the selected manually and continually to the surface water at these locations; and 1 had a surface- measured water levels was April 10, 2018 (table 2). For manu- water altitude that was much lower than the nearby well ally measured wells with one measurement, the only available screened in the MRVA aquifer, possibly because the aquifer Data and Methods 5 40 -- 119 135 138 471 223 1,126 map, spring 2018 used to generate the Total number of wells Total potentiometric-surface 6 7 4 9 2 -- -- 28 map, spring 2018 continually and used in the potentiometric-surface Number of wells measured - ). -- 40 115 129 131 462 221 1,098 in the potentiometric- Number of wells mea surface map, spring 2018 sured manually and used Ladd and Travers, 2019 Ladd and Travers, 6 7 4 2 -- -- 11 30 Total number of wells Total measured continually, measured continually, pre-irrigation season, 2018 -- 41 129 137 121 462 254 1,144 Total number of wells Total irrigation season, 2018 measured manually, pre- measured manually, MAP Region Total number of wells that were completed in the Mississippi River Valley alluvial aquifer and measured manually one or more times continually from January number of wells that were completed in the Mississippi River Valley Total

St. Francis Cache Grand Prairie Delta Boeuf Atchafalaya Deltaic and Chenier Plains alluvial aquifer Valley Mississippi River Table 1. Table data were used to generate the potentiometric-surface map for through May (pre-irrigation season) 2018, and the subset of these wells whose groundwater-altitude spring 2018, by Mississippi Alluvial Plain region ( alluvial aquifer, Mississippi River Valley Alluvial Plain; --, no data] Mississippi [MAP, 6 Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018 -- Median 20180411 20180410 20180412 20180409 20180402 20180324 20180410

-- All wells 20180523 20180522 20180516 20180418 20180503 20180424 20180523 Maximum -- Minimum 20180305 20180320 20180124 20180225 20180205 20180226 20180124 -- -- Median 20180410 20180410 20180410 20180410 20180410 20180410 -- -- 20180410 20180410 20180410 20180410 20180410 20180410 Maximum Continually measured wells -- -- Minimum 20180410 20180410 20180410 20180225 20180409 20180225 ). Mississippi River Valley alluvial aquifer, spring 2018 alluvial aquifer, Mississippi River Valley -- Median 20180410 20180413 20180412 20180409 20180402 20180324 20180410 Ladd and Travers, 2019 Ladd and Travers, -- 20180523 20180522 20180516 20180418 20180503 20180424 20180523 Maximum Summary statistics for water-level measurement dates of water levels used in the potentiometric-surface map, Summary statistics for water-level monitoring networks -- Minimum 20180305 20180320 20180124 20180402 20180205 20180226 20180124 Manually measured wells in groundwater MAP Region Summary statistics for water-level measurement dates of water levels used in the spring 2018 potentiometric-surface map for wells that were completed Summary statistics for water-level

St. Francis Cache Grand Prairie Delta Boeuf Atchafalaya Deltaic and Chenier Plains alluvial aquifer Valley Mississippi River Table 2. Table alluvial aquifer and measured manually one or more times continually as part of groundwater monitoring networks from January through May Mississippi River Valley (referred to as spring) 2018, by Mississippi Alluvial Plain region ( Alluvial Plain; --, no data] Mississippi year; MM, month; DD, day; MAP, YYYY, YYYYMMDD format; [Minimum, maximum, and median columns are shown in Data and Methods 7

93 92 91 90 89 PLANATIN ILLINOIS 55 Mississippi Alluvial Plain MAP region 57 St. Francis 24 Cache 3 MISSOURI Grand Prairie KENTUCKY Delta Boeuf Atchafalaya e g id Deltaic and Chenier Plains R s y e Urban areas ARKANSAS l w o 36 r C 40 Areas of the MRVA aquifer with insufficient groundwater-altitude data, spring 2018 County or parish boundary Stream TENNESSEE Location and measurement date of wells used in the spring 2018 40 potentiometric-surface map, MRVA aquifer, by part of month 35 # Day 16 to end of month, January 2018 (1 well) MISSISSIPPI ! Day 1 to 15, February 2018 (1 well) # 30 Day 16 to end of month, February 2018 (6 wells) ! Day 1 to 15, March 2018 (76 wells) er iv # Day 16 to end of month, March 2018 (105 wells) R i ! p Day 1 to 15, April 2018 (706 wells) p i s # s Day 16 to end of month, April 2018 (154 wells) 34 i s ! 530 s i Day 1 to 15, May 2018 (61 wells) M # Day 16 to end of month, May 2018 (16 wells)

Location and measurement date for steamgages used in the spring 2018 55 potentiometric-surface map, MRVA aquifer, by part of month !< Day 16 to end of month, January 2018 (1 streamgage) 33 ^ Day 1 to 15, April 2018 (65 streamgages) Continuously measured well used in the spring 2018 potentiometric-surface map, MRVA aquifer 20 20

LOUISIANA 32

31 49 10 12 55 10 10

30 0 25 50 MILES

0 50 100 KILOMETERS Current extent of the MRVA aquifer (Painter and Westerman, 2018) GULF OF MEXICO MAP regions (Ladd and Travers, 2019) Well and streamgage locations and water-level-altitude measurement dates 29 (U.S. Geological Survey, 2018, 2019a, 2019b) ase from U.S. Geological Survey digital data, 1:1,000,000 and 1:2,000,000 variously dated Highways from National Highway Planning Network, various scales, 2014 Urban areas from U.S. Census ureau, 1:500,000, 2016 Albers National Hydrogeologic Grid proection Standard parallels 4530’ N. and 2930’ N., central meridian 9600’ W. World Geodetic Survey of 1984

Figure 2. Location of wells with groundwater-altitude values and streamgages with surface-water-altitude values used to create the potentiometric-surface map of the Mississippi River Valley alluvial (MRVA) aquifer, spring 2018, and the part of the measurement month for the selected water-level-altitude value. 8 Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

Table 3. Total number of streamgages in the Mississippi Alluvial Plain with surface-water-altitude values for April 10, 2018, and number of surface-water-altitude values, generally for April 10, 2018, used to generate the potentiometric-surface map, spring 2018, for the Mississippi River Valley alluvial aquifer by Mississippi Alluvial Plain region (Ladd and Travers, 2019).

[MAP, Mississippi Alluvial Plain; MRVA, Mississippi River Valley alluvial]

Number of streamgaging stations with surface- Number of surface-water-altitude values, generally for MAP Region water-altitude values, generally for April 10, 2018, April 10, 2018, and used to generate the potentiometric- in the MAP area surface map of 2018 for the MRVA aquifer St. Francis 5 5 Cache 19 15 Grand Prairie 3 2 Delta 11 6 Boeuf 21 15 Atchafalaya 24 23 Deltaic and Chenier Plains 21 0 MRVA aquifer 104 66 is confined at the well location. There were 66 streamgages groundwater data for 2018 to create a potentiometric-surface in areas with nearby groundwater-altitude data for 2018 map for spring 2018. The resultant spatial files are in Albers (defined as groundwater data within 12.4 mi of the center of equal-area conic projection in meters using the World the cell that contains the streamgages) used to create the 2018 Geodetic Survey of 1984 and the potentiometric-surface potentiometric-surface map (fig. 2). The daily mean value altitude is expressed relative to NAVD 88. The rasters have was retrieved for January 15, 2018, for 1 streamgage; April 8, a cell size of 1,000 meters and are aligned with the National 2018, for 1 streamgage; April 9, 2018, for 1 streamgage; and Hydrologic Grid (Clark and others, 2018). April 10, 2018, for 63 streamgages. When a daily mean value The steps used to generate the potentiometric-surface was not available for April 10, 2018, the available daily mean map for 2018 are described in the “Data and Methods” sec- value closest and prior to April 10, 2018, was retrieved and tion for the report on the 2016 potentiometric-surface map considered. The surface-water-altitude values were approxi- (McGuire and others, 2019) and within the metadata for the mations of the groundwater altitude because the altitude raster and contours of the 2018 potentiometric-surface map where the groundwater and surface are connected likely is (McGuire and others, 2020). A GIS was used to delineate the below the surface-water altitude and above the altitude of the areas with little groundwater-altitude data for 2018 (McGuire river bottom. and others, 2019); these areas total about 5,725 mi2, or about The groundwater- and surface-water-altitude data 13 percent of the MRVA aquifer area. were organized into two files McGuire( and others, 2020). The potentiometric-surface raster, which was output from Two fields were added to each groundwater- and surface- the interpolation process using the GIS command “topo to water-altitude record in the files—the “USE_2018” and raster,” was compared to a raster of the aquifer base (Torak “USECMT2018” fields. The “USE_2018” field was assigned and Painter, 2019) to identify where the potentiometric-surface a positive value for records with a water-level altitude that raster was below the aquifer-base raster. The potentiometric- was used in the creation of the potentiometric-surface map and surface raster was as much as 16 ft below the aquifer-base a negative value for records that were excluded. For records raster in an area of about 44 mi2 in the south-central part of where the “USE_2018” field is negative, the “USECMT2018” Lonoke County, Arkansas. No changes were made to the field is populated with a brief explanation for why the water potentiometric-surface raster as a result of this comparison level was not used for the 2018 potentiometric-surface map. because there were groundwater-altitude data for two wells thought to be screened in the MRVA aquifer, which indicates that the aquifer base warrants review for this area and that Characterizing the 2018 Potentiometric-Surface the well depth and screened interval for the two wells war- Raster and Contours rants investigation. The well identification code for these wells (termed “site badge” in the related data file;McGuire The potentiometric-surface raster and contours were and others, 2020) are USSCS:344411091505001 and generated using a series of GIS commands and source files of USSCS:344654091495401. selected groundwater- and surface-water-altitude data from A total of five potentiometric maps were created—one January to May 2018 (tables 1 and 3; McGuire and others, for the entire MRVA aquifer, and one each for the St. Francis 2020). About 87 percent of the aquifer area had sufficient and Cache MAP regions in the north, Boeuf and Grand Prairie Potentiometric-Surface Map, Spring 2018 9

MAP regions in the west-central area, Delta MAP region in potentiometric-surface map and were located in a raster cell the east-central area, and Atchafalaya and Deltaic and Chenier with a potentiometric-surface value, was 1.6 ft with a bias of Plains MAP regions in the south (Ladd and Travers, 2019). −0.35 ft. The maps are at a reduced, regional scale of 1:625,000 to The spring 2018 potentiometric contours in the Cache allow for the display of control-point values. region ranged from 110 to 340 ft above NAVD 88 and show The interpolation process, which was used to generate a large depression in the lower one-half of the Cache region the rasters, can result in cell values for cells collocated with (sheet 2). The lowest measured groundwater altitude was a measured well that are generally similar to, but commonly 108.50 ft in a depression in Poinsett County, Ark., and the not equal to, the corresponding groundwater- and surface- highest measured groundwater altitude was 339.30 ft in water-altitude values based on measurements. This difference Bollinger County, Mo.; the lowest measured surface-water is partly because the cell values represent the value for the cell altitude was 173.69 ft in Prairie County, Ark., and the high- area, and the measured values are values at specific locations est was 349.67 ft in Bollinger County, Mo. (McGuire and within the area represented by the cell. others, 2020). Flow in the Cache region generally is to the To assess the uncertainty in the final raster and con- south-southwest or into the depression in the southern part of tours, the water-level altitude for all values considered in the region. the potentiometric-surface map and used to generate the The spring 2018 potentiometric contours in the potentiometric-surface raster were compared to the final St. Francis region ranged from 170 to 320 ft above NAVD 88 potentiometric-surface raster value in the cell where the well (sheet 2). The lowest measured groundwater altitude was or streamgage is located (McGuire and others, 2020). The 163.00 ft in Cross County, Ark., and the highest measured root mean square error (RMSE) and bias were calculated groundwater altitude was 316.40 ft in Scott County, Mo.; the for the difference between the manually- or continually- lowest measured surface-water altitude was 274.83 ft in Lake measured water-level altitude and the value extracted from the County, Tennessee, and the highest was 316.91 ft in Alexander potentiometric-surface raster generated using the contours and County, Illinois (McGuire and others, 2020). Flow in the point values (Helsel and Hirsch, 2002). St. Francis region generally is to the south-southwest. The spring 2018 potentiometric contours in the Boeuf region ranged from 30 to greater than 230 ft above NAVD 88 (sheet 3). The lowest measured groundwater altitude was Potentiometric-Surface Map, Spring 27.38 ft in Concordia Parish, La., and the highest measured 2018 groundwater altitude was 215.57 ft in Pulaski County, Ark.; the lowest measured surface-water altitude was 36.46 ft The spring 2018 potentiometric-surface contours ranged in Concordia Parish, La., and the highest was 231.27 ft in from 10 to 340 ft above NAVD 88, and the regional direc- Pulaski County, Ark. (McGuire and others, 2020). Flow in the tion of groundwater flow was to the south-southwest, except Boeuf region is to the southeast, southwest, south, and into in areas of groundwater-altitude depressions (sheet 1), where the depressions. groundwater flowed into the depression, and near rivers, where The spring 2018 potentiometric contours in the Grand flow was generally parallel to the rivers. However, in some Prairie region range from 90 to 230 ft above NAVD 88; there areas, flow was from the aquifer into the river or from the river is a large depression in the potentiometric surface within the into the aquifer. The lowest measured groundwater altitude region (sheet 3). The lowest measured groundwater alti- was in Saint Landry Parish, Louisiana, and the highest was tude was 81.02 ft in Arkansas County, Ark., and the highest in Bollinger County, Missouri; the lowest measured surface- measured groundwater altitude was 228.46 ft in Independence water altitude was in Saint Mary Parish, La., and the highest County, Ark.; the lowest measured surface-water altitude was was in Bollinger County, Mo. (sheet 1; McGuire and others, 194.17 ft in White County, Ark., and the highest was 204.79 ft 2020). Based on groundwater- and surface-water-altitude mea- in White County, Ark. (McGuire and others, 2020). Flow in surements from January to May 2018, the MRVA aquifer is the Grand Prairie region generally is into the depression that connected to surface-water features in some areas and discon- encompasses most of the region. nected in other areas at least during part of the year; however, The spring 2018 potentiometric contours in the Delta the extent of the degree of connectivity of these areas cannot region range from 60 to 210 ft above NAVD 88; there is a be derived from these data. large depression in the potentiometric surface within central The RMSE for the differences between the measured part of the region (sheet 4). The lowest measured groundwater water-level altitude and potentiometric-surface-raster value altitude was 59.18 ft in Sunflower County, Mississippi, and for the 46 manually and 2 continually measured wells, which the highest measured groundwater altitude was 199.54 ft in were not used in the potentiometric-surface map and were DeSoto County, Miss.; the lowest measured surface-water located in a raster cell with a potentiometric-surface value, altitude was 90.72 ft in Warren County, Miss., and the highest were 21 and 2 ft, respectively. The RMSE for the difference was 211.79 ft in Shelby County, Tenn. (McGuire and others, between the measured water-level altitude for the 1,126 manu- 2020). Flow in the Delta region generally is into the large ally and continually measured wells, which were used in the depression at the center of the region. 10 Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018

For most of the Atchafalaya region and all the Deltaic and References Cited Chenier Plains region, a spring 2018 potentiometric-surface map could not be created because there were no groundwater- altitude data (sheet 5). In the part of the Atchafalaya region Ackerman, D.J., 1989, Hydrology of the Mississippi included in the 2018 potentiometric-surface map, potentiomet- River Valley alluvial aquifer, south-central United ric contours ranged from 10 to 50 ft above NAVD 88 (sheet 5). States—A preliminary assessment of the regional flow The lowest measured groundwater altitude was 4.07 ft in Saint system: U.S. Geological Survey Water-Resources Landry Parish, La., and the highest measured groundwater Investigations Report 88–4028, 74 p. 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Publishing support provided by the Rolla and Pembroke Publishing Service Centers McGuire and others—Altitude of the Potentiometric Surface in the Mississippi River Valley Alluvial Aquifer, Spring 2018—SIM 3453 ISSN 2329-132X (online) https://doi.org/10.3133/sim3453